A liquid crystal display apparatus such as a liquid crystal television receiver or a liquid crystal monitor is generally mounted with a backlight device that illuminates a liquid crystal display panel by a plurality of LEDs. In the backlight device, PWM control is performed that controls the duty ratio indicating the ratio of on period of an LED in a one-cycle control period to regulate the luminance of the LED.
Although in recent years a multiplicity of LEDs are disposed on the backlight device due to, e.g., the increased size of the liquid crystal display panel, the number of the LEDs is limited that can be connected in series at an output voltage of a power supply circuit supplying electric power to the backlight device. For this reason, a configuration is employed such that the plurality of LEDs are divided into a plurality of LED groups and connected in series and that the LED groups are connected in parallel to the power supply circuit (see, e.g., Patent Documents 1 and 2).
Patent Document 1 proposes to control the output voltage of the power supply device so that voltages at connection points between the LED groups and a constant-current output circuit are constant, to thereby supply a constant current required for the drive of the LED groups. Hereinafter, such a control is referred to as constant-current control.
In the configuration according to Patent Document 1, however, the output voltage becomes low when all the LED groups go out, so that when an LED group subsequently go on, it may be difficult to instantly supply a current required for the drive of the LED group.
It is thus conceivable to execute the constant-current control only when any one of the plurality of LED groups is on but to execute a constant-voltage control for controlling the output voltage so that the voltages applied to the LED groups become a predetermined value when all the LED groups are off. As a result of this, when an LED group starts to be turned on, the LED group can stably go on, while a required current can be supplied during the turning on of the LED group. The predetermined voltage value is set to a slightly higher level so that a sufficient current can be supplied at the start of turn-on of the LED group.